I was interested in this sessions mainly because I'm interested in where the short SF/F books went. The panelists included Martha Wells (moderator), Lillian Stewart Carl, Dee Beetem, Gail Dayton, and A. Lee Martinez.
Lillian, who is up for a Hugo later this summer, started the process with the obvious: epics and multi-book stories are publisher driven. Back in the day, Robert E. Howard wrote many stories about Conan but they were all stand-alones. All fantasy lives in the shadow of J. R. R. Tolkien. He wrote a multi-book epic with grand sweep (forget that the book is really one large book) and he set the bar and the mold. However, Tolkien was an exception. He wrote The Lord of the Rings without the market in mind. Indeed, there was not a market at all.
Dee brought up a good point: writers spend a lot of time building a world. Once built, it better pay off dividends. Lee stated that series is what people want. While he didn't come out and say it, I think series characters, like our TV series and late-night talk show hosts, are essentially comfort food. We want to return to that galaxy far, far away or read the latest Harry Bosch novel time and again.
The talk progressed to writing style. Lillian said that readers will be tolerant of bad writing if the story is good. (Kind of like Nora Roberts' first rule of writing I mentioned in the "Raiders of the Lost Maguffin" write-up)
Lee finds himself bored with the same old worlds. That's why he writes stand-alones and builds the world up every time. At this, Martha brought up another obvious point: readers nowadays are so sophisticated and they have so many genre tropes ingrained in our Reader DNA that a writer doesn't necessarily have to world-build the way they used to (like Edgar Rice Burroughs, for example). Again, Robert E. Howard was mentioned. He wrote what he knew. Sure, he used a different name for a four-legged animal you ride but its really a horse. All the panelists said they disliked wacky names for the sense of being wacky. I agree with them. Some of the names Alan Dean Foster used in his early books were a bunch of consonants. Lee said it best: we writers put too much pressure on ourselves to world build. It's not all that necessary to have the entire global structure in place before you write a story. You can world build with what's around the character and, as Martha said, you can fill in some gaps in the next book. Gail concluded about world building that you can explain as you go.
A funny aside: all the panelists, when they discussed Epic Fantasy, would take their hands and make an arc with one hand, almost like a rainbow. That morphed into visual code for "epic fantasy." It became the funny joke of the hour and Lee ran with it, visually describing other books with shorter arcs, choppy arcs, or none at all.
In summary, I think the panelists came to the same conclusion: sure, there is a market for massive, epic fantasy stories with tons of world building. But that's not the end-all, be-all in fantasy literature. There's more out there that's good.